The present invention concerns a method of making cold reduced Al-killed steel strips and sheet suitable for press forming by a continuous casting and continuous annealing process. More particularly, the present invention concerns a method of making a steel strip whose mechanical properties are not inferior to those of a steel sheet made by the batch type annealing process by means of controlling [N] %, Sol. Al. % and Sol.Al.%/[N]% ratio in the steel making stage.
A continuous casting process is advantageous in that the casting operation may be performed continuously with a consequent saving of manpower, the yield point is improved and uniform quality is obtainable. It has rapidly and widely come to be used in recent years. The steel widely used in continuous casting process for making cold reduced steel sheets is low carbon A1-killed steel having the general composition: C = 0.04 to 0.06%, Sol.AL. = 0.020 to 0.040%; N = 0.005 to 0.007%, Mn = 0.20 to 0.50%; P = 0.007 to 0.020%; and S = 0.010 to 0.030%.
Except for Sol. Al. and Mn, the above components are unavoidable in steel. The [N] is higher than that of Al-killed steel made in accordance with ordinary ingot making (0.003 to 0.005% [N]), because the molten steel is apt to come in contact with air more frequently between ladle and tundish, in the tundish and between the tundish and casting mold in the continuous casting process. The reason for controlling the Sol. Al content within the range of 0.020 to 0.040% is that advantageous results can be obtained in the form of steel sheet having excellent press-formability since the Lankford value is high and the yield point is low if the Sol. Al content is within said range. In the case of batch type annealing, when the aforementioned [N] range and Sol. Al range are used comparable results are not obtained. The reason for controlling the Mn content to 10 .times. [S]% to 0.50% is to avoid red shortness caused by S in steel by forming MnS.
The conventional continuous annealing process known in the art requires a shorter period of time for the steel to remain in the furnace than in the case of the batch type annealing process, and uses rapid heating and cooling. The steel sheet obtained by this annealing process has a low strain aging property and a high yield point and is not suitable for press forming. For these reasons, it is well known that it is mainly used for tin plate. However, the utility value of annealed cold reduced steel strip for press forming will be unlimited in view of the continuous annealing process, remarkable merits of the continuous annealing process for high efficiency in production and uniformity in quality once the above mentioned defects in the continuous annealing process have been rectified.
Until now the continuous process for obtaining a soft steel strip was proposed in U.K. Pat. No. 1,334,022 as a continuous annealing process for low carbon cold reduced soft steel strip for press forming which is characterized in that the cold reduced low carbon steel strip is heated, quenched, continuously passed through a furnace equipped with a heating zone, heated the strip up to 1,250.degree. to 1,300.degree. F. in said heating zone, quenched the same to below 1,000.degree. F. from the above temperature range in said quenching zone, e.g. at a rate of 50.degree. C.,/sec, and successively held the strip for at least 30 seconds within a temperature range of 800.degree. to 1,000.degree. F. in the shelf treating zone. However, the problem encountered in the above mentioned U.K. Patent process was that the shelf treatment continuous annealing given to continuously cast Al-killed steel having the above mentioned composition would never yield the steel sheet having the press formability of the commercial grade cold reduced steel sheet obtained by the batch type annealing process.
Generally, the properties required for a cold reduced steel sheet for press formability for it to be of commercial grade are as follows:
(1) it should have a low yield point (to be soft) and
(2) it should have excellent strain aging properties. The standards as regards the above mentioned properties of the commercial grade cold reduced steel sheet produced in the conventional batch type annealing process (Japanese Industrial Standards, G-3141, similar to ASTM A-109) are:
Yield Point = 22 to 23 kg/mm.sup.2
After tempering = reappearance of yield point
elongation after 38.degree. C. .times. 8 days aging is 1.5% or less.
If a steel continuously cast suitable for a continuous annealing process including shelf treating is developed under the present circumstances, process for the production of cold reduced steel strip from the steel making to the annealing is carried out by a continuous operation and its industrial merits will naturally be evaluated most highly.